Cryoprobes are known in the art for inducing a lower temperature or freezing in tissues. Typically, a cryogen is delivered into a cryoprobe in the form of mist, i.e., in the form of small cryogen droplets distributed in the vapors of the cryogen itself. A certain fraction of the liquid cryogen evaporates during delivery to the cryoprobe as a result of imperfections in the thermal insulation of a delivery hose. The cryogen mist cannot be separated completely in the internal cavity of the cryotip (the distal section of the cryoprobe) on the liquid and gaseous phases without application of special measures. Without such special means, it is impossible to use completely the liquid fraction of the cryogen for effective freezing.
There were some previous attempts with limited success to solve this problem. U.S. Pat. No. 5,324,286 describes a cryogenic apparatus which comprises a coolant system and a probe having a cryogenically-cooled tip. The probe is formed of an elongated housing having a distal end closed by the tip and a proximal end connected to the coolant system. The housing is adapted to receive cryogenic droplets entrained in a warm carrier gas stream supplied by the coolant system. The carrier gas stream passes through the housing such that the entrained cryogenic droplets are transported to the distal end of the probe for cooling the cold-tip. The tip is cryogenically-cooled by the cryogenic droplets which are collected at the base of the tip. More specifically, the carrier gas transports the entrained cryogenic droplets, through the inlet tube to the distal end of the probe where, because of their inertia, the droplets cannot follow the 180 degree bend of the returning carrier gas stream. Instead, the droplets are deposited and stored in a porous heat sink positioned in the cold-tip. The porous heat sink is positioned such that it is in thermal contact with a cold-tip head. Both the porous heat sink and the tip head are formed of a thermally conductive material. The liquid deposited in the heat sink from impinging droplets is evaporated by heat supplied by the object to be cooled, such as tumor tissue which is placed in contact with the cold-tip head. Accordingly, the tip reaches temperatures commensurate with the saturation temperature of the evaporating liquid cryogen.
U.S. Pat. No. 5,264,116 describes a cryoprobe with separation means in the form of a liquid nitrogen supply tube, which is provided with a plurality of small vent holes to vent gas formed or present in the refrigerant supply tube to the return refrigerant flow channel. The vent holes also allow a small amount of liquid nitrogen to vent into the return flow channel to further reduce the temperature differential between the sub-cooled liquid nitrogen supply and the counter-current flowing return refrigerant.
An analagous technical solution is described in U.S. Pat. No. 5,520,682. However, such design of a separator cannot ensure effective separation of liquid and gaseous phases of the cryogen mixture.
An article by S. L. Qi et al. “DEVELOPMENT AND PERFORMANCE TEST OF A CRYOPROBE WITH HEAT TRANSFER ENHANCEMENT CONFIGURATION” CRYOGENICS 46 (2006) 881-887, describes a cryosurgical system, which functions on the basis of liquid nitrogen, supplied into a cryoprobe from a dewar flask. In order to improve quality of the liquid-gaseous mixture supplied from the dewar flask, there is a separator, which is positioned immediately after the dewar flask and serves for separation between the liquid and gaseous phases of the stream.
However, this technical solution cannot provide complete separation of gaseous and liquid phases because of the process of further gasification of the liquid nitrogen, which occurs in the supplying hose of the system and in the cryoprobe itself as a result of imperfection of their thermal insulations.
U.S. Pat. Nos. 4,831,856 and 5,800,487, among others, describe application of helical tubes as counter-flow heat exchangers in cryosurgical instruments operating on the principle of Joule-Thomson.